The present invention relates to the field of casting, and more particularly to a casting tree for lost pattern casting, and also to methods of fabricating shell molds and to casting methods using such a pattern.
In the description below, terms such as “top”, “bottom”, “horizontal”, and “vertical” are defined by the normal orientation of such a mold while metal is being cast into its inside.
So-called lost wax or lost pattern casting methods have been known since antiquity. They are particularly suitable for producing metal parts with shapes that are complex. Thus, lost pattern casting is used in particular for producing turbine engine blades.
In lost pattern casting, a first step normally comprises making a pattern out of a material having a melting temperature that is comparatively low, e.g. out of a wax or a resin. The pattern is then incorporated in a casting tree that is then coated in a refractory material in order to form a mold. Among the various types of mold that can be used in lost pattern casting, so-called shell molds are known in particular, which are formed by dipping the pattern or a cluster of patterns in a slip, and then dusting the slip-coated pattern or cluster with refractory sand in order to form a shell around the pattern or the cluster, followed by firing the shell in order to sinter it so as to consolidate the assembly. Several successive dipping and dusting operations may be envisaged in order to obtain a shell of thickness that is sufficient before it is fired. The term “refractory sand” is used in the present context to mean any granular material of sufficiently fine grain size for complying with the desired production tolerances, that is capable in the solid state of withstanding the temperatures of the molten metal, and that can be consolidated into a single piece while firing the shell.
After emptying or eliminating the material of the pattern from the inside of the mold, whence the “lost pattern” name of such methods, molten metal is cast into the mold so as to fill the mold cavity left inside the mold by the pattern after it has been emptied out or eliminated. Once the metal has cooled and solidified, the mold can be opened or destroyed in order to recover a metal part having the shape of the pattern. In the present context, the term “metal” covers both pure metals and also, and above all, metal alloys.
In addition to at least one pattern, the casting tree normally comprises a part support comprising a distributor that is to form at least one casting channel in the mold, that is used to deliver molten metal into the molding cavity during casting. In order to be able to produce a plurality of parts simultaneously, it is possible to incorporate a plurality of patterns as a cluster in a single casting tree.
In order to increase rates of production, it is possible to fabricate not only the patterns but also the various elements of the part support by injection molding. Nevertheless, with the increasing complexity of individual patterns and of clusters of patterns in casting trees, the ability to assemble casting trees from a plurality of separate elements is becoming simultaneously more necessary and more difficult to perform. Typically, the various elements of a casting tree are assembled together manually and adhesively bonded together. However, given the increasing complexity of the tree and the ever-tighter tolerances for positioning and alignment, in particular when performing monocrystalline casting, that leads to ever-increasing costs and time for assembly.